1. Field of the Invention
The present invention relates to a method for adjusting an MRI (magnetic resonance imaging) superconducting magnet.
2. Description of the Background Art
In an MRI superconducting magnet, a static magnetic field in an imaging space is optimized at a design stage so that the uniformity of the static magnetic field in the imaging space will be in an order of ppm (parts per million).
In an actually manufactured MRI superconducting magnet, the uniformity of a static magnetic field is low, specifically, several hundred to several thousand ppm, due to a dimension error of each component and a manufacturing error such as an assembly error. In view of this, shimming is provided as an art of correcting such non-uniformity of static magnetic field.
A prior art document disclosing an MRI apparatus capable of shimming is Japanese Patent Laying-Open No. 2008-220923. In the MRI apparatus described in Japanese Patent Laying-Open No. 2008-220923, the non-uniformity of a static magnetic field is corrected by disposing iron pieces (iron shims) at desired locations.
Such MRI apparatuses improved in the uniformity of a static magnetic field by shimming are brought to and installed in use places such as hospitals. Magnetic field atmospheres in the use places in which the MRI apparatuses are installed differ from one another. Accordingly, the uniformity of the static magnetic field may be compromised by the magnetic field atmosphere in each use place. In view of this, a prior art document, Japanese Patent Laying-Open No. 02-083904, discloses a static magnetic field generating apparatus for reducing the influence of the magnetic field atmosphere in its use place.
In the static magnetic field generating apparatus described in Japanese Patent Laying-Open No. 02-083904, a magnetic plate is provided between a floor on which the MRI apparatus is installed and the MRI apparatus, so as to compensate the uniformity of the magnetic field. The influence on the static magnetic field mainly results from magnetic materials buried in the floor, such as iron bars and iron frames. In the MRI apparatus described in Japanese Patent Laying-Open No. 02-083904, the above-described magnetic plate is determined as an essential component for the MRI apparatus by factory default settings, thereby reducing the influence of the environment under the floor of the use place for the static magnetic field.
By disposing such a magnetic plate for compensating the uniformity of the magnetic field and magnetic members for shimming such as iron pieces in order to improve the uniformity of the static magnetic field in the MRI apparatus at the use place, electromagnetic force generated by an interaction between the magnetic plate and each of magnetic members is exerted on its MRI superconducting magnet.
Such an MRI superconducting magnet is capable of generating a strong and stable static magnetic field by maintaining a superconducting state. However, if an unacceptable amount of disturbance takes place in the superconducting magnet due to the influence of the above-described electromagnetic force, quench, a phenomenon of destroying the superconducting state, will be caused.
An MRI superconducting magnet is less likely to cause quench again in a condition in which it has caused quench once, i.e., exhibits a so-called “training phenomenon”. Conversely, when an MRI superconducting magnet is operated in a condition in which it has not been operated before, quench is highly likely to take place.